LOS ANGELES — The Mets might have their explanation for Jeremy Hefner’s brutal second half.

Hefner yesterday was diagnosed with a partially torn medial collateral ligament of the right elbow and placed on the disabled list, retroactive to Saturday. Because of the DL move, Hefner’s reassignment to Triple-A Las Vegas was nullified.

The Mets are calling the ailment a “chronic” condition that worsened as the season progressed. Hefner has a 9.13 ERA in five appearances since the All-Star break, after a dominant stretch to close the first half of the season.

Manager Terry Collins said it’s possible Hefner will pitch again this season. The right-hander has been told to rest and will be re-evaluated.

“This certainly had a lot to do with it,” Collins said before the Mets’ 5-4, 12-inning loss to the Dodgers last night. “I know he could pitch with it, that’s why he did, but we just saw a big difference that there had to be something that we tried to address.”

Collins said the organization was aware of Hefner’s elbow soreness since at least June.

“One of the things we talked about when we discussed a six-man [rotation] was there was so many other things involved,” Collins said. “[Hefner], Jon Niese, Zack [Wheeler]. It could have helped a lot of guys that extra day.”

* Wilmer Flores did not start again because of his sprained right ankle, but Collins said the infielder is “marginally better” and wanted to swing in the batting cage. Flores had a pinch hit single in the ninth and Jenrry Mejia pinch ran for him. Josh Satin started at third base last night.

“We’re going to be very careful about having [Flores] run on it,” Collins said.

The manager said if Flores still can’t run in the next few days, the DL likely would be imminent.

“If he recovers pretty fast he would be able to play I would think this weekend,” Collins said. “If not, then we have issues. I don’t think it’s anything that is going to be ultimately serious that he’s going to miss a real extended period of time unless we run him back out there too early and he twists it again.”

* Ike Davis was in the lineup against left-hander Chris Capuano. It was Davis’ first start against a lefty since his recall from Triple-A Las Vegas on July 5. He went 1-for-6 with a double, a steal and a run scored.